This invention relates to electronic identification systems and more particularly to such a system comprising an interrogator and a plurality of transponders.
In an electronic identification system of the aforementioned kind known to the applicant, the interrogator comprises a transmitter for transmitting an interrogation signal to a plurality of transponders and a receiver for receiving intermittently repeated discrete response signals from the transponders. A microprocessor in. the interrogator reads and identifies a particular transponder from characteristic data in a data stream in the response signal received from that transponder. In a time window immediately after transmission of the response signal, the transponder awaits an acknowledgement from the interrogator indicative of the fact that the transponder has been read by the interrogator. The acknowledgement is non-discriminative or non-specific and is provided by the interrogator by a brief interruption of the energizing signal, If the interruption is detected by a transponder during the aforementioned time window associated with that transponder, that transponder accepts that it has been read and it then changes to a sleep mode wherein it does not reply to the interrogator, to enable the interrogator to read other transponders not yet read. The probability of collisions in response signals from the plurality of transponders is reduced by utilizing for each transponder a respective randomly generated hold off time after energization by the interrogator has commenced and before the first response signal is transmitted by the transponder; and by causing the response signal to be repeated after randomly generated time periods.
In spite of these measures designed to avoid collisions and to facilitate accurate reading and subsequent switching of transponders already read, errors still occur. For example, because the acknowledgement signal is non-specific, any and all transponders the time window of which is active at the time when the acknowledgement signal is transmitted will be switched off, whether they have been read or not.
Accordingly it is an object of the present invention to provide an alternative system and method with which the applicant believes the aforementioned disadvantages may at least be alleviated.
According to the invention there is provided an electronic identification system comprising an interrogator and a plurality of transponders
the interrogator comprising:
transmitter means for transmitting an interrogation signal to the transponders;
each transponder comprising:
signature generating means for generating a unique signature
characteristic of the transponder;
transmitter means for intermittently transmitting in response to the interrogation signal a response signal sing the signature;
the interrogator further comprising acknowledgement signal generating means for generating upon reception of a response signal from one of the transponders, an acknowledgement signal comprising the unique signature of the transponder to be transmitted by transmitter means of the interrogator, thereby to acknowledge reception of the response signal to said one transponder.
The transmitter means of the transponder may comprise modulator means for backscatter modulating the interrogation signal with the response signal. In other embodiments the transmitter means may include a local carrier generator for generating and transmitting the response signal.
Further according to the invention each transponder may comprise time window generating means to generate a time window immediately after transmission of each of the intermittent response signals; and a controller for causing the transponder to change from a normal operational mode to another mode, if an acknowledgement signal comprising the respective signature is received during said time window.
The signature generating means may comprise a random number generator and the signature may be in the form of a randomly generated number. The generator may be adapted to generate a new signature each time the interrogation signal is received again, after it has been interrupted.
The random number generator may also be operative to determine a random delay or hold off period after first reception of the interrogation signal and before a first of said intermittent response signals is transmitted.
Furthermore, the random number generator may also be operative to determine random repetition periods for the intermittent response signals.
The random number generator may also be operative to generate a divisor for a programmable clock frequency divider of the transponder, from time to time to yield a randomly selected clock frequency for the controller of the transponder.
The aforementioned acknowledgement signal may also include a command to cause the transponder to switch from the normal operational modes to a selected one of a plurality of other modes.
Also included within the scope f the present invention is an interrogator for use with an electronic identification system also comprising a plurality of transponders; the interrogator comprising:
transmitter means for transmitting an interrogation signal to the plurality of transponders;
receiver means for receiving response signals originating from the plurality of transponders; each response signal comprising a signature characteristic of a respective transponder from which it originates; and
a controller for processing at least one of the response signals received and for causing an acknowledgement signal comprising the signature to be transmitted.
Still further included within the scope of the present invention is a transponder comprising:
receiver means for receiving an interrogation signal;
signature generating means for generating a signature characteristic of the transponder;
transmitter means for transmitting in response to the interrogation signal a response signal comprising the signature characteristic of the transponder; and
a controller adapted to cause the transponder to switch from one mode of operation to another, if an acknowledgement signal comprising the signature is received.
The signature generating means may comprise a random number generator and the signature may thus be in the form of a number.
The transponder may also comprise time window generating means to generate a time window immediately after transmission of the response signal within which time window the acknowledgement signal is expected.
The number generator may be operative to determine a delay or hold off period after first reception of the interrogation signal and before the response signal is transmitted.
The random number generator may be operative to determine random repetition periods for the response signal.
The random number generator may be operative to generate a divisor for a programmable clock frequency divider of the transponder, from time to time to yield a randomly selected clock frequency for the controller of the transponder.
According to another aspect of the invention there is provided a method of operating an electronic identification system comprising an interrogator and a plurality of transponders the method comprising the steps of:
transmitting from the interrogator an interrogation signal to the transponders:
causing the transponders to receive the interrogation signal and each intermittently to respond with a response signal comprising a signature characteristic of the transponder;
causing the interrogator to receive the response signals and to process a response signal from at least one of the transponders;
causing the interrogator to transmit an acknowledgement signal comprising the signature of the at least one of the transponders; and
causing the at least one of the transponders upon reception of the acknowledgement signal to switch from one mode of operation to another mode of operation.